Silicon-based microlaser by doped thin films

ABSTRACT

A silicon-based microlaser formed of rare-earth-doped CaF 2  thin films has a semiconductor substrate material (240) and a CaF 2  film layers (234) grown on semiconductor substrate material (240), The CaF 2  film layer (234) is doped with a predetermined amount of rare-earth-dopant that is sufficient to cause a spectral emission from the CaF 2  film layer (234) having a narrow linewidth when the CaF 2  film layer (234) is optically or electrically pumped.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to lasers, and more particularly to asilicon-based microlaser formed from a doped thin glass film capable ofproducing an intense spectral emission having a narrow linewidth uponbeing optically or electrically pumped.

BACKGROUND OF THE INVENTION

The availability of all silicon-based optoelectronic integrated circuit(OEIC) technology promises to revolutionize the optoelectronic industryand significantly impact a wide range of both military and commercialapplications. One such area of impact is multi-chip moduleinterconnectivity. Silicon-based OEICs will not only solve resistivityand high-capacitance problems by replacing electron transport withphotons, they will also provide new functionality, such as circuit-levelimage processing. Silicon OEICs will also provide cost inroads tocommercial markets as high-volume silicon processes enjoy economies ofscale unparalleled by other electronic or optoelectronic materialstechnologies. Furthermore, silicon-based OEICs are expected to providenew functionality such as circuit-level image processing.

There are four technologies required to make silicon-based OEICs areality: (1) detectors; (2) waveguides; (3) modulators and (4) emitters.While there has been considerable progress in the first three areas, alack of an appropriate silicon-based light-emitting device, particularlya silicon-based laser, has greatly hindered the development of fullyintegrated silicon-based OEICs technology.

Most work to date on OEICs has been based on III-V materials. However,post-ultra large-scale integrated (ULSI) work will likely continue touse silicon substrates because of low material costs, high mechanicalstrength, good thermal conductivity, and the highly developed processingmethods available for silicon. One approach to integrating optical anddigital electronics is to integrate III-V silicon materials usingepitaxially grown III-V layers for selected regions on siliconsubstrates. Although laser action from III-V layers grown epitaxially onsilicon has been demonstrated, progress in this area has been limited bymaterial quality problems resulting from the large lattice and thermalexpansion mismatch between the two systems and incompatibilities betweenIII-V and silicon processing.

Reduced cavity size has been found to significantly affect lasercharacteristics for silicon-based lasers. When the cavity length iscomparable to the wavelength of the cavity-defined radiation,cancellation of spontaneous emissions, zero-threshold lasing andenhanced gain may be achieved. The degree of gain enhancement isdetermined by the coherent length of the spontaneously emittedradiation. Gain enhancement has been predicted to increase more thanfive-fold in III-V semiconductor microcavities as the emission linewidthdecreases from 100 nm to 30 nm.

Although possible applications of these phenomena have been consideredmostly for semiconductor lasers, the microcavity effects can also beapplied to solid state lasers. Solid state lasers generally providebetter thermal stability and narrower emission linewidths thansemiconductor lasers. The narrow emission linewidths gain media fromsolid state lasers are ideal for inducing microcavity gain enhancementeffects.

A need also exists for a way, to achieve light emission from these filmsby electroluminscence rather than by photoluminescence. If this werepossible, electrons could be used to produce photons and, thus, utilizethe optoelectronic devices made of these films through voltagevariation.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided asilicon-based microlaser formed with Nd³⁺ doped a CaF₂ thin film. Themicrolaser includes at a minimum a semiconductor substrate materialcomprising silicon and a CaF₂ film grown on the semiconductor substratematerial. The CaF₂ film includes a predetermined amount of Nd³⁺ dopantsufficient to cause a strong spectral emission when the CaF₂ film isoptically pumped.

A technical advantage of the present invention is that it usesrare-earth-doped CaF₂ as a gain medium for the silicon-based microlaser.As a result, the present invention possesses significant advantages,including high-intensity emission even for films as thin as 200 nms;narrow emission linewidths (e.g., 0.12 nm at 4.2 at 1 nm at 2980 K), theability to use established growth processes, and to patternsubmicrometer features.

Another technical advantage of the present invention is that thesilicon-based microlaser has a very low lasing threshold. This isadvantageous because first, if laser action can not be obtained bydirect electrical pumping of the gain-medium, the low thresholdexhibited by microcavities allows pumping by weak electroluminescentlight sources. Additionally, low thresholds are attractive for low-powerapplications, particularly in future high-density circuits where powergeneration will be performance limiting.

Another technical advantage of the present invention is that sincesolid-state lasers are generally less sensitive to temperature changesthan semiconductor lasers, the silicon-based microlaser of the presentinvention will be more stable than semiconductor lasers operating inclose proximity to high-power-density circuitry.

Another technical advantage of the present invention is that since CaF₂may be grown on silicon epitaxially, and vice-versa, a solid state laserthat uses CaF₂ as the host material is compatible with silicon-basedtechnology,

Yet another technical advantage of the present invention is due to theproperty that RE-doped fluorides also exhibit very narrow emissionlinewidths. In particular, photoluminescence spectra from silicon-basedCaF₂ :Nd microcavity layers formed according to the present inventionshow high intensity and narrow emission linewidths, even when thethickness of the CaF₂ :Nd films is reduced to 0.2 microns.Photoluminescence linewidths from the CaF₂ :Nd films of the 0.12 nm and1.5 nm have been observed for spectra taken at 4.2 K and 300 K,respectively.

Another technical advantage of the present invention is that thephotoluminescence intensities of CaF₂ :RE films used in thesilicon-based laser of the present invention are relative insensitive tothe crystalline quality of the films. This is significantly differentfrom semiconductor laser materials, whose photoluminescence intensity ishighly dependent on the crystalline quality of the materials.Consequently, semiconductor laser materials are more vulnerable todefects in the materials. The bright and narrow emission lines obtainedfrom submicrometer-thick CaF₂ :Nd films, plus their low refractive indexand insensitivity to crystalline defects, provide an ideal material forachieving a silicon-based microcavity laser.

A technical advantage of the present invention is that it permits theuse of several approaches to increase electroluminscence efficiency ofrare-earth doped CaF₂ thin films. These include (1) semi-conductive CaF₂films grown by sputtering; (2) forming superlattice structures made ofCaF₂ :RE and other semiconductor layers that cause carriers toaccelerate in semiconductor lasers and gain enough energy to induceexcitation in the CaF₂ :RE layers; and (3) adding a second RE as anactivator together with a primary center (such as Nd and Eu) into CaF₂.

An important aspect of the preferred embodiment takes the form ofsilicon-based microlaser by doped thin glass films. The preferredembodiment employ rare-earth-doped CaF₂ thin films. By monolithicallyintegrating optical and electrical components on a single semiconductorchip, optoelectronic integrated circuits (OEIC's) offer great potentialto meet future telecommunication and computing system needs. Most ofcurrent OEIC's are made of III-V compounds. Silicon-based OEIC's takeadvantage of the well-established silicon technology and reduce thematerial cost of OEIC's. However, the development of silicon-basedOEIC's has been hindered by the lack of an appropriate silicon-basedlight emitting device-particularly, a silicon-based laser device.Although GaAs on silicon has been used to make light emitting devicesrecently, the crystalline quality is still immature for a laser deviceat this moment.

Since CaF₂ can be grown epitaxially on silicon and vice versa, a solidstate laser made of CaF₂ will be compatible with silicon-basedtechnology. Furthermore, a solid state laser offers narrower emissionlines and better thermal stability than a semiconductor laser. Bulk CaF₂:Nd and Nd-doped CaF₂ thin films grown according to the preferredembodiment on silicon and Al/Si exhibit strong photoluminescenceemission lines. These results indicate that these thin films can be usedfor the fabrication of a silicon-based light emission device and asilicon-based microlaser chip.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows the gain enhancement of a microcavity with several emissionlinewidths ranging from 30 nm to 100 nm which are typical for III-Vsemiconductor;

FIGS. 2 and 3 show the helium temperature photoluminescence spectrum ofa 1.0 micron thick CaF₂ :Nd film grown a Al(111)/Si(111) substrate;

FIG. 4 shows concentration dependence on the luminescence from a onemicron-thick CaF₂ :Nd film grown on Al/Si(111);

FIG. 5 shows the relative intensity of the 10457 Å line for CaF₂ :Ndfilms grown on Al/Si(111) with various thicknesses;

FIGS. 6 and 7 show the room-temperature photoluminescence spectra of a0.2 micron thick CaF₂ :Nd film (1%) and a 4.0 micron thick GaAs film;

FIG. 8 shows photoluminescence spectra from CaF₂ :Nd on a Ta₂ O₅ /SiO₂distributed Bragg reflector;

FIG. 9 shows the photoluminescence spectra from a half-wavelength CaF₂:Nd//(Ta₂ O₅ /SiO₂) microcavity at room temperature;

FIG. 10 illustrates the calculated reflectivity spectrum from ahalf-wavelength CaF₂ :Nd//(Ta₂ O₅ /SiO₂) microcavity using aone-dimensional model;

FIG. 11 shows a typical photoluminescence spectrum of porous silicon;

FIGS. 12 and 13 show two embodiments of the present invention thatintegrate porous silicon with CaF₂ :Nd so that porous silicon can beused as the light source to pump a CaF₂ :Nd gain medium;

FIGS. 14 and 15 show two alternative embodiments of the presentinvention that are similar to the embodiments of FIGS. 15 and 16;

FIG. 16 shows a superlattice structure that can be used to obtaineffective electroluminescence from a CaF₂ :ZnS multilayer structure;

FIGS. 17 and 18 illustrate conceptually two embodiments of anelectrically-pumped quarter-wavelength shifted DFB laser using RE-dopedCaF₂ as the gain medium; and

FIGS. 19 and 20 illustrate two structures that use electroluminescentZnS as a light emitting source and CaF₂ :RE as a gain medium.

DETAILED DESCRIPTION OF THE INVENTION

The development of microcavity lasers originated from studies ofinteractions between atoms/molecules and the electromagnetic radiationfield. In 1946, Purcell predicted that the spontaneous emission rate ofan excited atom would be changed if the atom were put in a cavity withdimensions similar to the radiation wavelength of the atom. Usingfluorescent dyes, the phenomenon was demonstrated experimentally byDrexhage in 1974. In 1981, the issue was readdressed by Kleppner, and anincreasing number of experimental and theoretical results were reportedin the following decade. Recently, increasing interest in thefabrication of surface emitting laser devices, which are inherentlycomposed of high-quality cavities and often radiation wavelengths ofsimilar dimension, has notably escalated the studies of microcavityeffects.

For a conventional laser, the efficiency to couple emitted radiationinto a mode is on the order of 10⁻⁴ to 10⁻⁵. When the optical modes arereduced to single mode by the formation of a microcavity, the couplingefficiency can be close to unity. Other unique features of a microcavitylaser include thresholdless lasing, disappearance of relaxationoscillation, increase in dynamic response speed, and altered emissionlifetime. The degree of microcavity effects is dependent on the emissionlinewidth of the gain medium, mirror reflectivity, difference inrefractive indices between the gain medium material and the mirrormaterials, position and thickness of the gain medium, and the threedimensions of the cavity.

Microcavity effects have been calculated to increase as emissionlinewidth decreases. FIG. 1 shows the gain enhancement of a microcavitywith several emission linewidths ranging from 30 nm to 100 nm, which aretypical for Group III-V semiconductors. In particular, graph 50 of FIG.1 shows along ordinate 52 a measure of gain enhancement versusreflectivity as plotted along abscissa 54. Exemplary linewidths areshown as line 56 for a 30 nm linewidth, line 58 for a 62.5 nm linewidth,and line 60 for a 100 nm linewidth. The gain enhancement for an emissionwith a 100-nm linewidth saturates at 2.4 when reflectivity is nearunity. In contrast, the gain enhancement is 7.6 when the emissionlinewidth is 30 nm.

Microcavity effects are also strongly dependent on the quality of thecavity mirrors- FIG. 1 also shows that high-reflectivity mirrors providehigh gain enhancement, especially when the emission linewidth is small.When a mirror is made of quarter-wavelength multilayer dielectric film(i.e., a distributed Bragg reflector (DBR)), the quantum efficiencyincreases as the difference in refractive index between the alternativefilms increases. In addition to the difference in refractive index,other factors such as control of film stoichiometry, interfacesharpness, and film stress may also affect the performance of themirrors.

The large difference in refractive index between CaF₂ and othersemiconductors has prompted efforts to use CaF₂ as the material forhigh-quality Bragg reflectors. For example, using Bragg reflectors madeof CaF₂ and ZnSe, a microcavity III-V semiconductor laser can be formedto exhibit laser performance that is superior in terms of thresholdcurrent and lasing efficiency to a laser using Si/SiO₂ mirrors.

Enhancement or prohibition of spontaneous emission in a microcavity canbe achieved by controlling the length of a microcavity and the positionof the active gain medium in the cavity. Enhanced emission occurs whenthe cavity length is tuned to the emission wavelength and the activegain medium is positioned at the antinode position. In thisconfiguration, the field will interact with the gain medium- Reducedemission will be observed when the gain medium is located at a nodeposition.

Theoretical analysis of a three-dimensional tetragonal microcavityconfined normal to the surface by DBRs and transversely by cleavedplanes suggests that with an emission linewidth at 1.5 nm the efficiencyfor coupling total radiation energy into a resonant mode (spontaneousemission factor) increases more than an order of magnitude when thetransverse dimensions are reduced from 15 to 2 times the radiationwavelength. Similar enhancement in coupling efficiency has also beensuggested for a cylindrical microcavity structure.

For the preferred embodiment, Al and CaF₂ films were grown on 4-inchdiameter silicon wafers in an ultra-high vacuum system (VG Semicon V80)composed of a molecular beam epitaxy (MBE) chamber, a metallizationchamber and a chemical vapor deposition chamber. Wafers can betransferred between these three chambers through a ultra-high vacuumtransfer system, which is annexed with two loading locks. Base pressureof the MBE chamber and the metallization chamber were below 1×10⁻¹⁰ mbarand 1×10⁻⁹ mbar, respectively. The chamber pressure during CaF₂ growthwas 5×10⁻¹⁰ mbar and the process pressure during Al growth was 2×10⁻⁹mbar. CaF₂ and NdF₃ are evaporated thermally from effusion cells. Thedeposition rates of these fluorides are determined by controlling thecell temperatures and monitoring the pressure of the fluxes. Thedeposition rates increase proportionally as the flux pressures increase.The compositions of the films are determined by an X-ray fluorescencemethod. The thickness of the films are decided by stepprofile-measurements. CaF₂ :Nd films are grown on Si(111) andAl(111)/Si(111) substrates with substrate temperatures from 100° C. to700° C. According to the results from photoluminescence (PL) spectra,the optimal growth temperature for CaF₂ :Nd growth is 500° C. on siliconand 100°-300° C. on Al.

Characterization of the optical properties of the films byphotoluminescence (PL) show strong PL emission at ˜0.9 μm, ˜1.3 μm, and˜1.1 μm, including the 1,046-μm laser emission line observed for bulkCaF₂ :Nd. As the thickness of the films decreases from 1 μm to 0.2 μm,the intensity decreases proportionally, and the emission wavelengths andthe relative band intensities of the PL spectra from the CaF₂ :Nd filmsgrown on Al/Si remain unchanged. This result suggests that theluminescence properties of the films are not distorted, even though the0.2-μm film is close to the CaF₂ /Al interface where more defects tendto exist.

It is important to note that transition elements, instead of rare-earthelements may be used as dopants for the purposes of the presentinvention. Additionally, PL intensities of CaF₂ :Nd films do not dependon the crystalline quality of the films. This is very different fromsemiconductor light-emitting materials, whose PL intensities are highlydependent on the crystalline quality of the materials. Consequently,they are more vulnerable to defects in the materials.

FIGS. 2 and 3 show the helium temperature PL spectrum of a 1.0-μm thickCaF₂ :Nd (0.48 wt %) film grown on a Al(111)/Si(111) substrate. In FIG.2, chart 64 has a possible intensity measurement range of 0.000 through0.140 as arbitrary units along ordinate 66 and wavelength measurementsranging from 1040 nm through 1100 nm along abscissa 68. L centeremission as indicated by luminescence spike 68 has a linewidth of 0.12nm. Other luminescence peaks include peak 70, 72 and 74. The transitionsshown in FIG. 2 are from ⁴ F_(3/2) →⁴ I_(11/2) states.

Chart 76 of FIG. 3 includes a possible intensity measurement range from0.000 through 0.020 in arbitrary units along ordinate 78 plotted againstthe wavelength range from 8500 to 9300 Å along abscissa 80. Luminescencepeaks include approximately equivalent intensity peaks 82 and 84,smaller luminescence peaks 86 and 88, and yet smaller intensity peaks 90and 92. The peaks in FIG. 3 result from ⁴ F_(3/2) →⁴ I_(9/2)transitions. The emission wavelengths observed from L centers in bulkCaF₂ :Nd are marked with arrows. Spectra from a 10 μm thick 0.3 wt. %CaF₂ film on a CaF₂ substrate, show similar wavelengths and relativeintensities to those in FIG. 3.

The similarities of PL spectra for CaF₂ :Nd grown by different methodsand on different substrates suggest that, at low Nd concentrations,emission from L centers dominate. In contrast, as the Nd concentrationincreases, more significant differences in PL spectra are observed.

FIG. 4 shows concentration dependence of the luminescence from1-μm-thick CaF₂ :Nd films grown on Al/Si(111). Chart 100 of FIG. 4illustrates intensity in arbitrary units ranging from 0.000 through0.600 along ordinate 102 plotted against wavelengths in Angstromsranging from 10400 through 10600 Å along abscissa 104. In particular,plot 106 records luminescence intensity for a 1-μm-thick CaF₂ :Nd filmwith 0.48% Nd formed on a MBE-grown layer of Al/Si (111). Plot 108 plotsluminescence intensity for the same thickness of CaF₂ :Nd film with0.96% Nd on Al/Si(111). Plot 110 shows luminescence for the samethickness of CaF₂ :Nd film with 1.9% Nd on Al/Si(111). Plot 112 showsluminescence intensity for the same CaF₂ :Nd film thickness with 3.8% Ndon Al/Si (111). The 10457 Å line, which is usually used for laseroperation completely disappears in bulk material when the Ndconcentration is above 3.8 wt. %.

The most important difference between the current films and previouslyreported bulk CaF₂ :Nd and CaF₂ :Nd/CaF₂ is the reduced intensity ofemission lines from M and N defect centers in the current films evenwhen the Nd concentration is as high as 3.8 wt. %. For bulk CaF₂ :Ndwith 3.8 wt. % Nd, the emission intensity from M and N centers are aboutthe same as that from L centers. For CaF₂ :Nd/CaF₂ with at least 3.7 wt.% Nd, the intensity of the luminescence at N centers is 80% theintensity at L centers. In contrast, FIG. 4 shows that no emission atthe N center wavelength (10448 Å) is detected in any of the CaF₂ :Nd onAl/Si samples. As the Nd concentration increases, a small peak appearscorresponding to the M center (10467 Å) but the intensity is still only50% of the intensity of the L center at a 3.8 wt. % Nd concentration.

These comparisons indicate that for the current CaF₂ :Nd grown at lowtemperatures by MBE the orthorhombic N and M centers formed byaggregated Nd³⁺ -F⁻ can be reduced. The low substrate temperature usedto grow CaF₂ :Nd/Al/Si may be the reason why low M and N defect CaF₂ :Ndcan be prepared. Low growth temperature may prevent fluorine loss and,thus, avoid the charge compensation mechanism in bulk CaF₂ :Nd. Besides,the distribution of Nd-F and its aggregates can be altered by thelow-temperature process, because the kinetic energy of the atoms is toolow for the atoms to diffuse and reach an equilibrium state as occurs attemperatures above 600° C.

FIG. 5 shows the relative intensity of the 10457 Å line for CaF₂ :Nd(1.9 wt. %) films grown on Al/Si(111) with various thickness and a 0.2μm CaF₂ :Nd (1.9 wt. %) film grown on Si(111). The plot 134 of FIG. 5records along abscissa 136 intensity in arbitrary units potentiallyranging from 0.0 through 1.2 with CaF₂ :Nd film thickness measurementsranging from 0.0 through 1.2 μm. Thus, with a thickness of 0.2 μm, therelative intensity for an CaF₂ :Nd film on Al/Si(111) substrate isrecorded as point 140. For a Si(111) substrate relative intensity isrecorded at point 142 for a 0.2-μm CaF₂ :Nd film thickness. Other filmthicknesses and relative intensities for two CaF₂ :Nd film thicknessinclude a 0.5-μm film thickness at point 144 and 1.0-μm thickness atpoint 146.

The PL intensity decreases proportionally as the thickness of the filmsreduces from 1.0 μm to 0.2 μm. Furthermore, the emission wavelengths andthe relative PL intensity of the films are identical regardless of theirthickness. These results suggest that luminescence properties are notdistorted even when the film is only 0.2 μm, although the 0.2-μm-thickfilm is close to the CaF₂ /Al interface and tends to be more vulnerableto interfacial defects. For films of the same thickness, but differentsubstrates, the PL intensity of the CaF₂ :Nd on Al is higher than thatof the CaF₂ :Nd on Si. The higher PL intensity from CaF₂ :Nd onAl/Si(111) may be because over 90% of the 514.5 nm incident light isreflected into the CaF₂ :Nd film from the Al layer, while the light ismostly absorbed in the Si substrate of CaF₂ :Nd/Si(111).

FIGS. 6 and 7 show the room-temperature PL spectra of a 0.2-μm-thickCaF₂ :Nd (1%) film and a 4.0-μm-thick GaAs film, respectively. Inparticular, plot 120 of FIG. 6 records along ordinate 122 relativeintensity within a range from 0.00 through 0.45 in arbitrary units alonga wavelengths range from 6,000 through over 13,000 cm⁻¹ along abscissa124. PL intensity peaks occur in regions 126, 128 and 130. Withinregions 126, 128, and 130, local specific frequencies associated withthe identified intensity peaks are identified by associated wavelengthsin plot 120 of FIG. 6.

In FIG. 7, plot 150 shows a range for relative intensity of from 0.000through 0.450 along ordinate 152 with wavelengths in Angstroms rangingfrom less than 6,000 to greater than 13,000 cm⁻¹ along abscissa 154.Within this range, peak 156 shows a relative intensity of approximately0.40 at a L center of 11,423 cm⁻¹. The CaF₂ film is grown on aAl(111)/Si(111) substrate. The GaAs film is grown on a Si(100)substrate. It is clearly shown that the linewidth of the CaF₂ :Nd laseremission line (9560 cm⁻¹) is narrower than the linewidth of the 11423cm⁻¹ GaAs peak.

The intensity of the CaF₂ :Nd emission line in FIG. 6 is as strong asthe intensity of the GaAs peak in FIG. 7, even though the CaF₂ thicknessis only one-twentieth of the GaAs thickness. This indicates that CaF₂:Nd films can be a good gain medium for laser emission. The crystallinequality of the films are characterized by X-ray diffraction and exhibita full-width-half-maximum of 130 arcseconds from the GaAs film. Incontrast to GaAs, whose PL intensity strongly depends on the crystallinequality of the material, the PL intensity of CaF₂ :Nd does not showdetectable dependence on the crystalline quality of the film. Similarroom temperature PL spectra from polycrystalline and single crystal CaF₂:Nd films of the same thickness and Nd concentration have been observed.

Comparison of these CaF₂ :Nd samples with the high quality GaAs filmgrown on Si(100) (X-ray rocking curve halfwidth of GaAs=130 arcsec)shows that the PL-intensity from a 0.2-μm CaF₂ :Nd(0.38 wt %)/AlSi thinfilm is as high as that of the 4-μm-thick GaAs/Si sample, while incidentlight intensity is identical. The linewidths from the CaF₂ :Nd films are1.2 Å and 15 Å when the spectra are taken at 4.2 K and 300 K,respectively. In comparison, the PL linewidths from the GaAs/Si sampleare 40 Å and 220 Å at 4.2 K and 300 K, respectively. The narrow emissionlinewidth of the CaF₂ :Nd films and the small refractive index of CaF₂are attractive characteristics of a microcavity laser using the conceptsof the present invention. Good optical gain may be achieved because ofthe narrow linewidth. Also, high reflectivity mirrors can be fabricatedby taking advantage of the large difference in refractive index betweenCaF₂ and semiconductors.

In summary, thin CaF₂ :Nd films grown epitaxially on Al/Si(111)according to the preferred embodiment exhibit strong photoluminescenceemission even when the film thickness is reduced to 0.2 μm. Suppressedluminescence from M and N centers can be obtained from thelow-temperature-grown CaF₂ :Nd films. The emission line at 10457 Å,which was used for laser operation from bulk CaF₂ :Nd, shows very narrowlinewidth and does not quench until the Nd concentration exceeds 3.8 wt.%. These features makes that CaF₂ :Nd thin films an attractive gainmedium for a microcavity laser fabricated on Si-based substrates.

Yet another aspect of the preferred embodiment is the formation of CaF₂:Nd microcavity devices. High-intensity PL obtained from polycrystallineCaF₂ :Nd forms the basis of microcavity structures composed ofpolycrystalline CaF₂ :Nd films sandwiched by two DBRs (distributed Braggreflectors) to illustrate the invention concepts of the preferredembodiments. The DBRs for these configuration are made of ten pairs ofTa₂ O₅ /SiO₂ multilayers. Four-inch silicon wafers were used as thesubstrates.

Shown in FIG. 8 is a room-temperature PL spectrum taken from a CaF₂ :Ndfilm grown on a DBR. For FIG. 10, plot 190 records intensity inarbitrary units ranging from 0 through 4 along ordinate 191 againstwavelengths in Å ranging from 8000 to approximately 15000 Å alongabscissa 192. Peak regions of interest include peak region 194 and minorpeak regions 196 and 198. No mirror was fabricated on the top of theCaF₂ :Nd film. The spectrum shows very similar emission wavelengths andrelative intensities to those observed from CaF₂ :Nd films on Si(111)and Al(111)/Si(111).

FIG. 9 shows a room-temperature PL spectrum of a microcavity structuremade of CaF₂ :Nd and Ta₂ O₅ /SiO₂ multilayers. Plot 200 of FIG. 9records intensity ranging from 0 to 4 in arbitrary units along ordinate202 versus wavelength in Å ranging from 8000 to approximately 15000 Åalong abscissa 204. Within plot 200, peak 206 shows the room-temperaturePL spectra that the microcavity structure yields. The structure is thesame as the sample used for FIG. 14, except that a top DBR mirror hasbeen added to the structure, to form a one-wavelength CaF₂ :Nd//(Ta₂ O₅/Si)₂ cavity.

Comparing FIGS. 8 and 9, strong emission lines at 10453 Å and 10472 Åare detected in FIG. 9, while other emission lines around 10500 Åcompletely disappear. Furthermore, the intensities of the emission linesaround 9000 Å and 13000 Å are observed to become much smaller in respectto the 10460 Å transition. These results indicate that the radiationrate at 10453. Å and 10472.1 Å are enhanced by the microcavity and thetransitions around 9000 Å and 13000 Å are quenched. Intensityenhancement and linewidth reduction also observed for PL measurements at770 K.

Shown in FIG. 10 is the calculated reflectivity spectrum for theone-wavelength CaF₂ :Nd//(Ta₂ O₅ /SiO₂) microcavity structure. Thestructure is designed to form a resonant mode at 10460 Å. FIG. 10 showsthat while the emission lines around 1050 nm (⁴ F_(3/2) →⁴ I_(11/2)transition) are in the "stopband," the transitions around 900 nm (⁴F_(3/2) → ⁴ I_(9/2) transition) and 1300 nm (⁴ F_(3/2) ?⁴ I_(13/2)transition) are in the "passband" of the mirror structure. Consequently,the emission lines around 9000 Å and 13000 Å will not be affecteddirectly by the reflectors and can be used as references when theirintensities are compared with those around 1050 Å.

The preferred embodiment also exploits the optical properties of otherRE-doped CaF₂ to use the effects of different emission wavelengths in amicrocavity. Rare-earth ions that luminesce in the visible region are aparticularly significant feature of the preferred embodiment. Althoughit is difficult to predict whether stimulated emissions can be obtainedfor given dopant transitions, the studies for rare-earth-doped bulk CaF₂and other bulk crystals with similar crystal field environments indicatea wide selection of other laser center dopants applicable to theconcepts of the present invention. Electroluminescence andphotoluminescence from CaF₂ thin films doped with Eu have emissionwavelengths centered in the blue at 420 nm, indicating that Eu-dopedCaF₂ can be a good gain medium. Intense electroluminescence in theultraviolet region with wavelengths as short as 306 nm have beenobserved in GdF3 doped ZnF₂ films. In addition to the Nd-doped CaF₂ thinfilms on silicon and Al/Si, Dy-doped bulk crystals may be used with thepreferred embodiment. Dy-doped material exhibits photoluminescenceemission features in the blue-green (around 486 nm), yellow (575 nm),red (665 nm) and near-infrared region of the spectrum (7540 Å and 8390Å). Ho and Dy are good doping candidates, respectively, for green andyellow emitters.

Instead of single crystal silicon, light emission from "porous Si" hasbeen achieved by both electrical and optical pumping. Integrating poroussilicon with Nd-doped CaF₂ provides a silicon-based material ("porousSi") with wide emission bandwidth as the light source and asilicon-based gain medium (CaF₂ :Nd) with narrow emission bandwidth forlasing. The emission wavelengths of porous silicon are between 5200 Åand 9000 Å. These wavelengths are ideal for pumping CaF₂ :Nd thin films.

Although porous silicon has been formed heretofore electrochemically,analytical studies of porous silicon show that this material is amixture of amorphous silicon, crystalline silicon, silicon oxides andsilicon hydrides The term "porous Si", thus refers to the siliconmaterial that is able to emit visible light. These emissions can beachieved not only by anodizing crystalline silicon, but potentially byconventional growth techniques such as chemical vapor deposition (CVD)and molecular beam epitaxy (MBE).

FIG. 11 shows a typical photoluminescence spectrum of porous silicon.Plot 216 of FIG. 14 records along ordinate 218 intensity in arbitraryunits ranging from 0 to 200,000 and along abscissa 220 wavelengthsranging from 5,200 to 9,000 Å. Curve 220 shows the luminescence rangefor porous silicon with a peak 224 between approximately 7,250 and 7,400Å. The luminescence of the preferred embodiment ranged from 5200 Å to9000 Å with a maximum intensity at around 7000 Å. Since the absorptionspectrum of CaF₂ :Nd shows strong peaks at 7250-7450 Å, 7800-8000 Å and8500-8700 Å, the photons emitted from porous silicon that can beeffectively absorbed by CaF₂ :Nd. This results in high-intensityphotoluminescence from CaF₂ :Nd.

FIGS. 12 and 13 show two structures that integrate porous silicon withCaF₂ :Nd. Structure 226 of FIG. 12 includes transparent electrode 228attached to porous silicon layer 230 that is formed upon the first setof Bragg reflectors 232. Next, CaF₂ :Nd layer 234 is sandwiched betweenBragg reflector set 232 and Bragg reflector set 236. Bragg reflector set236 is mounted to metal electrode 238 which has substrate 240 as itsbase. In these configurations, porous silicon can be used as the lightsource to pump CaF₂ :Nd. While only five layers of dielectrics are shownfor each Bragg reflector, the number of layers may be more than 10 pairsfor vertical surface emitting laser device so that high reflectivity canbe achieved. For edge emitting laser device, the requirement for highreflectivity is not as demanding as vertical surface emitting laser. Ametal layer or a Bragg reflector of small number of pairs will besufficient.

FIG. 13 shows another structure for porous Si as the light emittingsource beginning with transparent electrode 228 over porous Si layer 230which covers another transparent electrode 244. Attached to transparentelectrode 244 is a first Bragg reflector set 232 which attaches to CaF₂:Nd layer 234. CaF₂ :Nd layer 234 on its lower side attaches to Braggreflector set 236. In the configuration of FIG. 16, Bragg reflector set236 directly attaches to substrate 240. In FIG. 13 both porous siliconand CaF₂ :Nd are sandwiched between the two electrodes. This structureallows the CaF₂ :Nd layer to emit light by electroluminescence incertain cases. The CaF₂ :Nd layer will also be optically pumped by theelectroluminescence from porous Si. The drawback of the configuration inFIG. 16 is that the electric field gradient in the porous Si is not asstrong as that in the porous Si in FIG. 15. In FIG. 16, the CaF₂ :Ndlayer is optically pumped by the electroluminescence from porous Si. Noelectric field variation takes place in CaF₂ :Nd in this configuration.

FIGS. 14 and 15 show two other structures that are similar to FIGS. 14and 15, except that the porous Si is grown between CaF₂ :Nd and Sisubstrate. FIG. 14 shows yet a further embodiment 244 with porous Sibetween the CaF₂ :Nd gain medium and the substrate. In particular,transparent electrode 228 attaches to first Bragg reflector set 232which covers CaF₂ :Nd layer 234. CaF₂ :Nd 234 attaches to Braggreflector set 236. In the embodiment of FIG. 14, the porous Si layer 230is positioned between second Bragg reflector set 236 and metal electrode238. Metal electrode 238 attaches between porous Si layer 230 andsubstrate 240.

FIG. 15 also shows an embodiment 246 such that porous Si layer 230 ispositioned between the CaF₂ :Nd gain medium having CaF₂ :Nd layer 234and substrate 240. Particularly, Bragg reflector set 232 forms over CaF₂:Nd layer 234 cover a portion of Bragg reflector sets 236. Under Braggreflector set 236 is metal electrode 238 that attaches to porous Silayer 230. A second metal electrode 248 separates porous Si layer 230from substrate 240. In comparison with the structures in FIGS. 12 and13, the structures in FIGS. 14 and 15 make it difficult to grow singlecrystal materials on the top of the porous Si. However, these structuresare more attractive for a surface emitting laser, because the porous Siis not located in the path of laser output. This increases the outputefficiency of the laser device. Since the photoluminescence intensityfrom single crystal CaF₂ :Nd is comparable to that from polycrystalline,it may not be necessary to obtain single crystal layers on the top ofthe porous Si for lasing.

Several approaches can be used to increase the electroluminescenceefficiency: (1) Semi-insulating CaF₂ films can be grown by sputtering;(2) Superlattice structures made of CaF₂ :Nd and other semiconductorslayers can be used so that carriers will accelerate effectively in thesemiconductor layers and gain enough energy to induce Nd excitation inthe CaF₂ :Nd layers; and (3) Nd and another dopant such as Eu can beadded together into CaF₂. Since CaF₂ :Eu emits light at 420 nm byelectroluminescence, the Nd can be pumped by the photons emitted fromthe electroluminescence of Eu or even excited by direct energy transferfrom adjacent Eu atoms.

The following paragraphs discuss methods of the preferred embodiment toimprove efficiency of electroluminescence. Note, also, that thesemethods can be used in a laser pumped by electroluminescence. Thesuperlattice structure is similar to the cavity structure of amicrocavity laser. When the length of a laser cavity is of the sameorder as the emitted wavelength, the laser characteristics can besignificantly changed. In the microcavity domain, analysis andexperiments suggest that optical gain significantly increases as theemission linewidth decreases, particularly when mirror reflectivity ishigh. Given the very narrow linewidth from submicron thick CaF₂ :Nddescribed above and the fact that high reflectivity mirrors made of CaF₂/ZnSe have been demonstrated, a microcavity CaF₂ :Nd laser pumped byelectroluminescence may be achieved by improving the electroluminescenceefficiency with the following methods.

The first of the above methods for improving the efficiency ofelectroluminescence is through the use of sputtered CaF₂. While theresistivity of a CaF₂ film grown by evaporation is around 10¹⁶ ohm-cm, aCaF₂ film with resistivity as low as 3×10³ ohm-cm can be obtained bysputtering. The reduced resistivity is usually undesirable, because, inmost cases, CaF₂ has use as a good insulator that can be grownepitaxially on silicon or GaAs. However, good conduction properties areneeded if the film is used for electroluminescence applications. Since alarger number of carriers can be driven through semi-conductive relativeto insulating films, the collision cross-section between the carriersand luminescence centers in CaF₂ :Nd will be larger in thesemi-conductive sputtered films. This results in increased luminescenceintensity. Annealing CaF₂ :Mn in Cd vapor also increases theconductivity of the CaF₂ films. One possible disadvantage of thesputtered CaF₂ is enhanced emission quenching because of degradedcrystalline quality. However, this may not be a serious problem,because, as shown for MBE-grown CaF₂ :Nd, the photoluminescenceintensity of a polycrystalline CaF₂ :Nd film is the same as that of asingle crystal CaF₂ :Nd. This indicates that crystalline quality may notbe critical.

The second above-stated method for greater electroluminescenceefficiency is the formation of CaF₂ :Nd semiconductor superlattices.Superlattice structures made of CaF₂ :Nd and other semiconductors layerscan be used so that carriers will accelerate effectively in thesemiconductor layers and, thus, gain enough energy to induce Ndexcitation in the CaF₂ :Nd layers. This successfully improves theemission efficiency of a Y₂ O₃ :Eu thin-film electroluminescencedevices. Furthermore, when the thickness of each superlattice layers isless than the wavelength of the emission, constructive interference canbe produced to enhance the luminescence.

FIG. 16 shows a superlattice structure that can be used to obtaineffective electroluminescence from a CaF₂ :Nd/ZnS multilayer structure.The CaF₂ :Nd semiconductor superlattice structure 250 of FIG. 16includes transparent electrode 252 over CaF₂ :Nd/semiconductorsuperlattice 254. CaF₂ :Nd/semiconductor superlattice 254 includes CaF₂:Nd layers such as layer 256 interspersed with semiconductor layersusing, for example, ZnS to achieve the desired degree ofelectroluminscence. CaF₂ :Nd/semiconductor superlattice 254 is formedover metal electrode 238 which forms over substrate 240.

Transparent conductors can be made of conductive oxides, such as indiumtin oxide (ITO) or aluminum-doped zinc oxide, or heavily dopedsemiconductors such as ZnSe or ZnS. These materials offer goodelectrical conductivity and optical transparency at the emissionswavelength. Since the transparent conductors also act as mirrors toprovide optical feedback, the refractive index should be as different aspossible from that of CaF₂ :Nd (or other RE-doped CaF₂) so that strongmicrocavity effects can be achieved. The refractive index of ITO rangesform 1.75 to 2.3. The refractive indices of zinc oxide, ZnS, and ZnSeare around 1.86, 2.37, and 2.89, respectively, at 1-μm wavelength. Sincethe refractive index of RE-doped CaF₂ is about 1.43, these materialsshould be able to form high-quality DBR, or the periodic gain structurein DFB lasers, with the RE-doped CaF₂ to enhance the cavity effects.

Yet another method to improve electroluminescence efficiency as thoughthe formation of Co-doped CaF₂. Nd and another dopant, such as Eu or Er,can be added together into CaF₂. CaF₂ :Eu has been shown to emit lightat 420 nm by electroluminescence. When Eu and Er are co-doped with Nd inCaF₂, absorption at 398 nm and 449 nm for Eu and Er, respectively, hasbeen observed. These results suggest that, by growing a CaF₂ :Nd/Eu or aCaF₂ :Nd/Er film, the Nd can be pumped by the photons emitted from theelectroluminescence of Eu or Er, or even excited by direct energytransfer from adjacent Eu atoms to obtain enhanced luminescence.

Based on knowledge of the materials and features of a laser cavity,additional laser structures that are likely to achieve lasing byelectroluminescence are shown in FIGS. 17, 18, 19 and 20. The verticalconfiguration 260 of FIG. 17 includes transparent conductor 262 andmetal conductor 264 having a potential applied by independent voltagesource 266 to activate laser 268. Laser 268 includes interspersed layersof quarter-wavelength semiconductor 270 and quarter-wavelength CaF₂ :RElayers 272 to serve as the gain medium. Similarly, horizontalconfiguration 274 of FIG. 18 includes numerous transparent conductorssuch as transparent conductors 276 and 278 with CaF₂ :RE gain medium in280 interspersed therebetween.

FIGS. 19 and 20 illustrate two configurations that useelectroluminescent ZnS as a light emitting source as configurations 290and 291. Configuration 290 of FIG. 19 includes transparent electrode 292formed over ZnS layer 294. ZnS layer 294 serves as theelectroluminescent light source for Bragg reflector set 232, CaF₂ :RElayer 296 and second Bragg reflector set 236. Second Bragg reflector set236 forms over metal electrode 238 which attaches to substrate 240.Configuration 291 of FIG. 20 includes transparent electrode 292 which isformed over ZnS layer 294. ZnS layer 294 forms over a second transparentelectrode 298 which connects to Bragg reflector set 232. Between Braggreflector set 232 and Bragg reflector set 236 is CaF₂ :RE layer 296.Second Bragg reflector set 236 attaches directly to substrate 240 in theconfiguration of FIG. 23.

Extensive research has been conducted recently to achieve light emissionfrom group IV materials. Light emission from porous Si,isoelectronically doped silicon, and strained GeSi alloys have beenreported. Considering that these materials are compatible withsilicon-based technologies and that a very low lasing threshold may beachieved from the RE-doped CaF₂ microcavity laser, the preferredembodiment of the proposed microcavity laser readily accommodates thesematerials within the scope of the invention.

The availability of silicon-based OEICs and photonic integrated circuits(PICs) has a wide range of military and commercial uses. One applicationof silicon-based OEICs or PICs is in the area of multichip moduleinterconnectivity. As the complexity of multichip modules increases,interconnect resistivity and parasitic capacitance created byhigh-density metal lines pose major obstacles to system performance.Optical interconnectivity has long been considered a solution to theseinterconnectivity problems. Silicon-based OEICs and PICs, for example,have specific application for optically coupling phased-array radarelements. Whereas significant progress has been made in the area of GaAsand other III-V-based monolithic microwave integrated circuits (MMICs)for radar elements, work on digital logic and memory circuitry based onIII-V materials has not been as fruitful. Hence, optical sources andmodulators monolithically integrated with silicon digital circuitry mayprovide a mechanism for optical cooling state-of-the-art silicon digitaland logic circuitry to GaAs MMIC elements.

Current work in the area of optical computing is largely limited toIII-V and LiNbO₃ materials systems. With the exception of asilicon-based laser source, all other functions required for opticalcomputing (modulators, detectors, and waveguides) have beendemonstrated. In fact, large-scale integration (LSI) silicon detectors(e.g., CCDs) are commercially available, and medium-scale integration(MSI) modulators on silicon (e.g., DMDs) have been demonstrated and arebeginning to be commercially produced. Whereas DMDs are free-spacemodulators, materials such as BaTiO₃ and (Pb,La)(Zr, Ti)O₃ (which haveexcellent electro-optic properties) are used for high dielectriccapacitors in post-ULSI silicon memory applications that may be employedwith the various configurations of the preferred embodiment. SiO₂waveguides on silicon have also been demonstrated.

Silicon-based OEICs and PICs formed according to the concepts of thepreferred embodiment are particularly attractive, because existinghigh-volume silicon processes enjoy an economy of scale unparalleled byother electronic or optoelectronic materials technologies. Possibleapplications of silicon-based OEICs formed by the present invention aresilicon-based emitters for displays, fiber to the home and other cablenetworks, computer communication systems for automotive electronics, andlow-cost disposable OEICs for medical and in vivo uses. Finally,silicon-based OEICs according to present invention may provide newfunctionality such as circuit-level image processing devices or smartpixels.

In summary, concepts within the scope of the present invention include(1) preparation of thin CaF₂ :RE films; (2) fabricating optically pumpedsilicon-based CaF₂ :Nd microcavity lasers including the growth of CaF₂with other RE dopants, (3) growing and fabricating electroluminescentsources appropriate for pumping CaF₂ :RE microcavities, and (4)achieving laser action by either electroluminescent or direct electricalpumping.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A silicon-based microlaser formed of doped thinmaterial films, comprising:a semiconductor substrate material comprisingsilicon; at least one layer of CaF₂ grown on said semiconductorsubstrate material comprising a predetermined amount of dopantsufficient to cause spectral emissions on said layer when pumped; saidspectral emissions having a high intensity an narrow emissionlinewidths; said layer further having a thickness within an order ofmagnitude of the wavelength of said spectral emission such thatintensities of said spectral emissions are insensitive to crystalquality of the layer.
 2. The apparatus of claim 1, wherein said dopantcomprises a transition metal.
 3. The apparatus of claim 1, wherein saiddopant comprises a rare-earth element.
 4. The apparatus of claim 3,wherein said rare-earth element comprises Nd.
 5. The apparatus of claim3, wherein said rare-earth element comprises Eu.
 6. The apparatus ofclaim 1, wherein said dopant comprises a combination of Nd and Eu. 7.The apparatus of claim 1, wherein said dopant comprises a combination ofNd and Er.
 8. The apparatus of claim 1, wherein said layer is opticallypumped.
 9. The apparatus of claim 1, wherein said layer is electricallypumped.
 10. The apparatus of claim 1, further comprising a distributedBragg reflector set for reflecting said spectral emission.
 11. Theapparatus of claim 1, further comprising a distributed feedback laserfor reflecting and amplifying said spectral emission.
 12. The apparatusof claim 1 wherein said film quality of epitaxial.
 13. The apparatus ofclaim 12, wherein said dopant comprises a rare-earth element.
 14. Theapparatus of claim 13, wherein said dopant comprises Nd.
 15. Theapparatus of claim 1 wherein said film quality is polycrystalline.
 16. Amethod for producing a spectral emission comprising the steps of:forminga thin film layer of CaF₂ over a semiconductor substrate materialcomprising silicon, said thin film layer comprising a predeterminedamount of dopant; and pumping said thin film layer to produce a spectralemission having a narrow linewidth wherein said thin film layer has athickness within an order of magnitude of the wavelength of saidspectral emission, and wherein the intensity of said spectral emissionis generally insensitive to whether said thin film layer is epitaxial orpolycrystalline.
 17. The method of claim 16, wherein said step ofpumping said thin film layer comprises pumping a thin film layer havinga dopant comprising a transition metal.
 18. The method of claim 16,wherein said step of pumping said thin film layer comprises pumping athin film layer having a dopant comprising a rare-earth element.
 19. Themethod of claim 18, wherein said step of pumping said thin film layercomprises pumping a thin film layer having a dopant comprising Nd. 20.The method of claim 18, wherein said step of pumping said thin filmlayer comprises pumping a thin film layer comprising a dopant of Eu. 21.The method of claim 18, wherein said step of pumping said thin filmlayer comprises pumping a thin film layer comprising a dopant of Er. 22.The method of claim 16, wherein said step of pumping said thin filmlayer comprises pumping a thin film layer having a dopant comprising acombination of Nd and Eu.
 23. The method of claim 16, wherein said stepof pumping said thin film layer comprises pumping a thin film layerhaving a dopant comprising a combination of Nd and Er.
 24. The method ofclaim 16, wherein said step of pumping said thin film layer comprisesoptically pumping said thin film layer.
 25. The method of claim 16,wherein said step of pumping said thin film layer comprises electricallypumping said thin film layer.
 26. The method of claim 16, furthercomprising the step of forming a distributed Bragg reflector adjacentsaid thin film layer for reflecting said spectral emission.